This invention relates to a lithium secondary battery and a transportation method thereof excellent in safety and suppression of the highest temperature increase of the battery so as to avoid accidents like explosion, fire, and the like even in the case where the battery temperature is increased following abrupt battery energy discharge due to internal short circuit and external short circuit or in the case where the battery temperature is increased due to an unexpected situation during transportation.
Recently, a lithium secondary battery has widely been employed as a power source battery for portable electronic appliances such as a mobile phone, a VTR, a note type computer, and the like. Moreover, the lithium secondary battery has about 4V voltage for a single battery, which is higher than the output voltage of a conventional secondary battery such as a lead-acid battery, and also high energy density, so that the lithium secondary battery attracts attention to the application possibility as a power source, other than for portable electronic appliances, for driving a motor of an electric vehicle (EV) or a hybrid electric vehicle (HEV), which is positively suggested to be commonly used as a low pollution vehicle being backed by a matter of recent environmental pollution. Further, application to a power source for electric installation such as a headlight and a powered window of a vehicle is taken into consideration.
In a lithium secondary battery, generally, a lithium-transition metal compounded oxide is used for a positive active electrode material, a carbon material for a negative active electrode material, and a non-aqueous electrolyte solution containing an organic solvent and a lithium ion electrolyte dissolved in the solvent for an electrolyte solution. Electrode bodies which perform the battery reaction may have various shapes such as a sandwich type, a wound type, a laminated type, and the like and with any structure, a negative electrode and a positive electrode are separated from each other with a separator.
Regarding a battery for EV or HEV, since high power is required to drive a motor, a relatively high capacity is necessary for a single battery. It is therefore preferable to use wound or laminated type electrode bodies for such applications and to produce these electrode bodies, electrodes (meaning a negative electrode and a positive electrode) to be employed are produced separately by forming a positive active electrode material layer on the surface of a current collector substrate generally made of a metal.
In this situation, if internal short circuit, external short circuit or overcharge occurs in a lithium secondary battery comprising wound or laminated electrode bodies, the temperature of the battery is increased owing to Joule equivalent heat generated attributed to the inner resistance of the electrode bodies. The temperature increase is naturally intense in the case of abrupt large current flow in an electrode body and it possibly results in an accident of explosion of a battery and may further result in a disaster.
The causes of temperature increase of a battery are understood to include inner causes and outer causes. For example, the inner cause supposedly includes the case that a metal waste, which is highly electrically conductive, is mixed in during the assembly and penetrates a separator if there is damage in the separator and in any case, the electrode plates are short circuited between them to cause high electric current flow. The Joule equivalent heat generated at that time heats and evaporates the non-electrolyte solution to increase the pressure within the cell, resulting in possible occurrence of a fire or an explosion of the battery.
On the other hand, as an external cause, a case that a nail or the like, which is highly electrically conductive, penetrates the inside of a battery may be a possible cause, and also in that case, the same phenomenon as that of the inner short circuit occurs. Further a case that short circuit occurs between a positive electrode terminal and a negative electrode terminal is a probable cause and in this case, the degree of the heat generation differs based on the extent of the load (resistance) at the time of external short circuit. Other than that, the external cause includes a case that overcharging occurs owing to a problem with a charging apparatus, or a case that the battery is put near a heat radiating apparatus such as an engine and heated.
The inventors of the present invention have studied the various types of causes of the temperature increase of a battery and published the results of temperature alteration of the battery of a nailing test, an external short circuiting test, an overcharging test, and an external heating test for a lithium secondary battery having 25 Ah capacity in The Journal of Power Sources, 81-82 (1999) pp. 887-890. Among the tests, the inventors observed that the highest temperature increase was caused in the nailing test, that is, in the case of internal short circuit occurrence and a temperature increase as high as to about 400xc2x0 C. was observed.
Such a lithium secondary battery with a high capacity is equipped with a pressure relief valve to release the inner pressure of the battery when the battery""s inner pressure is increased to a prescribed pressure to prevent an explosion attributed to the temperature increase of the battery. However, in the case where the increase of the battery""s inner pressure is too sharp for the pressure relief to follow or where the pressure relief valve malfunctions, the explosion of the battery can not be avoided. Further, since the more fully the battery is charged, the more the energy to be discharged is increased, the temperature increase attributed to short circuit is considerable. Consequently, if the temperature of a battery is increased by some cause or other during the transportation of an assembled battery in a highly charged state, e.g. a fully charged state, from an assembling plant in one country to another place in the same country or to another country, the possibility of occurrence of an accident or hazard is increased.
The inventors of the present invention paid attention to the notion that though there are many causes of temperature increase of a battery, they are almost all attributed ultimately to the heating of the battery itself by the energy accumulated in the battery and that the temperature of the battery is most increased at the time of occurrence of an inner short circuit. In other words, the inventors have supposed that the temperature increase of the battery can be suppressed to a prescribed temperature or lower by satisfying prescribed relations between the energy quantity potentially accumulated in the battery and the specific heat (or the thermal capacity) of the battery itself, and have achieved the present invention.
The present invention provides a lithium secondary battery comprising an electrode body obtained by winding or laminating a positive electrode and a negative electrode via a separator, and a non-aqueous electrolyte solution, wherein the battery satisfies the following Equation (1):
E0/Cp+T1=T2 less than txe2x80x83xe2x80x83(1)
where E0 (J/g) is energy quantity per unit weight at the time of full charging of the battery; Cp (J/xc2x0 C.xc2x7g) is the specific heat of the battery; T1, (xc2x0 C.) is a normal use temperature of the battery; T2 (xc2x0 C.) is the highest elevated temperature of the battery; and t (xc2x0 C.) is the lowest temperature at which the battery falls into an unstable state.
The present invention also provides a method of transporting a battery comprising an electrode body obtained by winding or laminating a positive electrode and a negative electrode via a separator, and a non-aqueous electrolyte solution, wherein the battery is transported in a state in which the following Equation (2) is satisfied:
E/Cp+T3=T2 less than txe2x80x83xe2x80x83(2)
wherein E (J/g) is accumulated energy quantity per unit weight of the battery; Cp (J/xc2x0 C.xc2x7g) is the specific heat of the battery; T3 (xc2x0 C.) is a normal transportation temperature of the battery; T2 (xc2x0 C.) is the highest elevated temperature of the battery; and t (xc2x0 C.) is the lowest temperature at which the battery falls into an unstable state.
In this case, the highest elevated temperature T2 is preferably the boiling temperature of the non-aqueous electrolyte solution or lower. Also the highest elevated temperature T2 may be the boiling temperature of a component having the lowest boiling temperature among the main components of the non-aqueous electrolyte solution or lower. Further, the highest elevated temperature T2 is also preferably the melting temperature of a component having the highest melting point among the main components of the separator or lower.
The present invention is suitable to a lithium secondary battery with a battery capacity of 2 Ah or higher at the time of full charge and to transportation of such a lithium secondary battery. Further, a lithium secondary battery and its transportation method of the present invention are suitable to be employed for an electric power source for an electric vehicle or a hybrid electric vehicle.